Preparation of high-purity Ga₂O₃ by urea/ammonia-induced crystallization of GaOOH and its thermal conversion

Abstract

Gallium oxide (Ga2O3) is an important semiconductor material. Achieving high-purity Ga2O3 is a prerequisite for its applications. GaOOH is the key intermediate for Ga2O3 preparation. Herein, we propose a new method to prepare high-purity Ga₂O₃ based on α GaOOH crystallization followed by its thermal conversion. Urea/ammonia mixture is used as inducing agent for the first time, to precipitate Ga3+ in the dissolution liquor of β-Ga₂O₃ with purity of 99.7%(<3N). The gradual hydrolysis of urea and ammonia generates OH- at 85-95°C, leading to the slow precipitation of colloidal hydroxide that gradually converts to crystalline α GaOOH with purity of 99.9995%(>5N) during the subsequent 6 h aging process. Rectangular and spindle-like α GaOOH are obtained at initial solution pH of 3 and 10, respectively. X-ray absorption fine structure analysis demonstrates that the obtained α GaOOH has an octahedral structure of Ga³⁺ coordinated by three O²⁻ and three -OH with Ga–O bond length of 1.96 Å and 1.94-1.95 Å, respectively. Density functional theory calculations show that α GaOOH with the most stable configuration exhibits consistent adsorption behavior at different sites on the (110) crystal face, with weak adsorption to Na and Pb, and almost no adsorption to Co and Ni, which contributes to the suppression of impurities co-precipitation. The improvement in purity is mainly attributed to the precise control of α GaOOH crystallinity by adjusting urea/ammonia ratio, aging temperature and stirring speed. After calcination of α-GaOOH at 850 °C, β-Ga₂O₃ with purity of 99.9997% (> 5N) is obtained while maintaining the purity and morphology of the α-GaOOH. This study gives a facile route for the green, efficient and controllable preparation of high-purity Ga₂O₃.